1. Field of the Invention
The present invention relates to an electrostatic image forming method, and more particularly, to a method of forming an electrostatic image on insulating surface portions, which are formed in a pattern-like manner on an electrostatic image forming material, together with conductive layer portions, and to an apparatus therefor.
2. Description of the Prior Art
In the electrostatic image printing field, charging the insulating surface portions in addition to the insulating layer portions on an electrostatic image forming material has been widely accomplished. In this conventional method, more specifically, the electrostatic image forming material, on which the conductive layer portions and the insulating surface portions are distributed in a pattern-like manner, is placed on a conductive base plate. Then, charges are applied to the insulating portions, for instance, using a corona charging process, with the conductive portions being grounded to the earth, thus forming the desired electrostatic image.
Where an electrostatic image forming material which has a conductive intermediate layer formed on an insulating support and insulating layer portions formed on the conductive intermediate layer in a pattern-like manner is used in the conventional method, it is easy to ground the conductive layer to the earth with satisfactory results.
On the other hand, another method has been proposed, in which an image forming material which has conductive layer portions and insulating surface portions both distributed in a pattern-like manner is used, thus dispensing with the conductive intermediate layer on the insulating support.
In accordance with the above method, a method has been proposed, in which a conductive layer capable of being peeled off is formed on the surface of an insulating support. Desired portions of the conductive layer are then removed in a pattern-like manner using a stencil pen or the like so as to form recessed and exposed insulating surface portions on the surface of the insulating support. Charges are applied to the engraved insulating surface portions using a corona charging process or the like to thereby form a desired electrostatic image thereon.
In this method, moreover, when the electrostatic image on the insulating surface portions is intended to be transferred by applying toners thereto, the toner image obtained is formed on the engraved surface portions, so that the toner image will scarelly be damaged even if registration in the transfer operation is carried out. Therefore, this method is quite important in obtaining satisfactory transfer quality.
However, the method, in which the corona charging process is applied to the electrostatic image forming material formed with the conductive layer portions and with the insulating surface portions without the formation of the conductive intermediate layer on the insulating surface, is inevitably accompanied by the following drawbacks.
Reference will now be made to FIGS. 1 to 4 of the accompanying drawings. In FIG. 1a, reference numeral 1 indicates an electrostatic image forming material (which will be referred to hereinafter, for brevity, as a "material"), which is composed of conductive layer portions 4 in which at least the surface is conductive and formed on an insulating support 2 and of insulating surface portions 3 formed in a pattern-like manner on the insulating support 2. The insulating surface portions 3 are formed in such a fashion that a conductive layer 4, which is formed on the surface of the insulating support 2 to constitute the conductive layer portions 4, is removed in a pattern-like manner mechanically or chemically so as to expose the desired surface portions to the outside.
For illustrative purposes only, the insulating surface portions 3 are shown in FIG. 1b to have a shape of a modified letter "Q". In this case where the letter "Q" is engraved, the insulating surface portions 3 are divided into the following portions:
a. The portion 3' which is surrounded by conductive layer portion 4" around the letter;
b. The portion 3" which is surrounded both by the conductive layer portion 4" and by island-shaped conductive layer portions 4' electrically isolated from the portion 4"; and
c. The portion 3'" which is surrounded both by the portion 4" and by the portions 4'.
The "material" 1 as above is placed on a conductive base plate 5, which is grounded to the earth as shown in FIG. 1a, with its back face down. A corona charging process is then carried out by moving a corona charger 6, which is connected with a power source 7 and whose corona electrode 6' is impressed with a high DC voltage, above the "material" 1 in the direction of the arrow. If the conductive layer portion 4" around the letter "Q" is grounded to the earth in the manner as shown in FIGS. 1a and 1b, the charging operation is effected by the following actions.
At an initial stage of the charging operation, the corona ions are substantially uniformly applied to the surface of the "material". When the charging operation proceeds, the electric field of the "material" will reach the condition as shown in FIG. 2. In FIG. 2, the broken line arrows will indicate the behavior of the corona ions irradiated from the electrode 6' of the corona charger 6.
Among the ions thus uniformly applied, the ions applied to the surrounding conductive layer portion 4" will be neutralized because the portion 4" is grounded to the earth, whereas the ions applied to the insulating surface portions 3 and the island layer portions 4' will be stored temporarily therein as shown by the plus signs. The ions thus stored will exhibit a blocking action to succeeding corona ions, which are approaching to be applied to the "material" as shown by the broken line arrows. These charges establishing the blocking or repulsing electric field will be referred to, for brevity, as "blocking charges". Especially, the ions, which are approaching the vicinity of the insulating surface portions 3' and 3" having both of its sides or one of its sides grounded to the earth, will be subjected to the repulsive force of the ions having the "blocking charges " and accordingly will charge their courses such that they will be caught by the conductive layer portion 4" and neutralized.
On the contrary, the ions, which are approaching both the conductive island layer portions 4' electrically isolated from the surrounding layer portion 4" and the insulating surface portion 3"' surrounded by the portions 4', will be further trapped therein because they are separated from the grounded conductive layer portions 4" and accordingly because they cannot leak thereto. The trapping or storing action of the charges will continue until they build up sufficiently as "blocking charges" against corona ions coming, or until an equilibrium condition is attained with the potential of the electrode 6' of the corona charger 6. As a result, the charges stored in the conductive island layer portions 4' and in the insulating surface portion 3'" surrounded thereby will be increased to raise the potentials of the portions 4' and 3'". On the other hand, as these potentials become higher and higher increasing their repulsive forces, it becomes more and more difficult for corona charges to approach the insulating surface portion 3", which are defined both by the conductive island layer portions 4' and by the grounded conductive layer portion 4".
The charges thus obtained are illustrated in FIG. 3 in terms of the distribution of the surface potentials.
In FIG. 3, the abscissa is taken from the A - A crosssection of FIG. 1b, and the ordinate indicates the surface potentials of the respective portions. As shown, the potential in the grounded conductive layer portion 4" surrounding the letter "Q" is zero, and the potentials in the conductive island layer portions 4' isolated from the layer portion 4" have a certain level because the charges impinging upon the layer portions 4'are temporarily stored therein and cannot leak to the layer portion 4".
That portion 3' of the insulating surface portions 3 forming the electrostatic image, which is surrounded by the grounded conductive layer portion 4", has certain potential at its center, but this potential decreases in value toward the periphery until it becomes zero at its peripheral edges. This is because, during the charging operation, the ions leak to the conductive layer portion 4" which is grounded to the earth. On the other hand, in the insulating surface portion 3" which is defined by the conductive island layer portions 4' and by the grounded conductive layer portion 4", the coming corona ions are repulsed by the blocking voltage of the conductive island portions 4', so that the surface portions 3" have a potential of the certain level at the periphery of the island portions 4' but a potential which decreased toward the periphery of the grounded conductive layer portion 4" until it becomes zero at the peripheral edge of the portion 4". On the contrary, the insulating surface portion 3'", which is srrounded by the conductive island layer portions 4', has a uniform potential of the certain level.
As shown in FIG. 3, the level of the uniform potential of the insulating surface portion 3'" is much higher than the maximum level of the insulating surface portion 3', which is surrounded by the conductive layer portion 4". This is confirmed by the experimental results, as shown in FIG. 4. FIG. 4 is a graphical presentation, in which the surface potentials both of the insulating surface portion 3' surrounded by the grounded conductive layer portion 4" and of the insulating surface portion 3"' defined by the conductive island layer portions 4' are plotted against the impressed voltage. The potential of the insulating surface portion 3' is, as can be understood from this graph, saturated at a certain level. On the contrary, the potential of the insulating surface portion 3'" defined by the conductive island layer portions 4' is increased continuously with an increase in the impressed voltage of the corona charging operation.
As is apparent from the foregoing descriptions, the following disadvantages will arise, in the case where an electrostatic image is formed in the insulating surface portions of an electrostatic image forming material which has electrically isolated conductive layer portions on its surface.
First of all, the electrostatic image obtained has a limited low potential. This means that a visual image of high density cannot be obtained after the electrostatic image is developed.
Secondly, the distribution of the charges in the insulating surface portions forming the electrostatic image cannot be made uniform. This phenomenon results from the fact that the potential at the peripheral edge of the insulating surface portion adjoining the conductive layer portion will have a low level. After having been subjected to a developing treatment, the electrostatic image thus obtained cannot produce a clear visual image of high contrast.
Thirdly, the potential of the insulating surface portion will have different levels depending upon whether the conductive layer portions surrounding the insulating surface portions are grounded or not. The electrostatic image thus obtained will produce a nonuniformity in the visual image.
In this connection, it has been proposed in Japanese Pat. No. 43-14846 that all of the island portions be grounded to materially eliminate the island-shaped conductive layer portions.
According to this proposal, a conductive fine net pattern is formed before or after the formation of an insulating image on and in contact with the conductive layer portions so as to eliminate the island-shaped conductive layer portions. In this method, therefore, if the charging operation is carried out with any point of the conductive layer portions being grounded, then the charges, which are temporarily stored in the conductive layer portions as a result of the impingement of the corona ions, are neutralized to form an electrostatic image corresponding to the insulating image. However, this method has the following inherent drawbacks, that is:
a. The electrostatic image obtained is often damaged partially by the net pattern and becomes difficult to read out; PA1 b. Since commonly such island-shaped conductive layer portions of various sizes are formed in a mixed way, the net pattern has to be produced with meshes of smaller size than that of the smallest conductive layer portion so that the smallest portion may be electrically connected with adjacent portions. From a technical standpoint, the electrostatic image obtainable is inevitably divided into elements which are so excessively fine that they become difficult to read; PA1 d. as has been discussed as the first disadvantage of the conventional method, the potential of the electrostatic image obtained cannot be made sufficiently high with a charging operation of a predetermined quantity.
c. An additional step for forming the net pattern is inevitably incorporated; and
A novel method as disclosed in U.S. Pat. application Ser. No. 482,869, filed June 25, 1974 simultaneously herewith has been developed which can form a uniform and dense electrostatic image on the image portions of the insulating surface without being accompanied by the above-itemized drawbacks, even with use of a material whose conductive layer portions are isolated from each other by insulating surface portions. This electrostatic image forming method comprises: uniformly charging the surface of an electrostatic image forming material having on its surface both (1) image portions each having an insulating surface and (2) non-image portions each having a conductive surface, in a manner that the conductive non-image portions are electrically isolated from the earth; and then eliminating the charges from the conductive non-image portions.
The present invention is directed to an improvement in this method described above. More specifically, where a plurality of conductive layer portions, which are isolated from each other, are not subjected simultaneously to a grounding operation after they are charged, a condition will arise in which some of them are grounded while others are not. As a result of this condition, a steep potential gradient is established between the grounded conductive layer portions having a zero potential and the conductive layer portions left ungrounded and having a high potential due to the applied corona ions. If the gradient is quite large in this instance, "atmospheric discharge" or "creepage discharge" will take place between the grounded conductive layer portions and the ungrounded conductive layer portions. Under this particular conduction, it is possible that the charges, which are stored in the insulating surface portions, might be induced by the discharge between the conductive layer portions to accomplish their own discharge to such an extent that their potentials are lost, thus damaging a portion of the electrostatic image obtainable.